What If? Lab

What if animals did not hibernate?

Some animals sleep through the winter. Their hearts beat slowly and they use very little energy. What if they stayed awake instead? Let's imagine it with Olly!

Winter brings two big problems for animals: less food and lots of cold. Bats, groundhogs, dormice and bears get through it by slowing their bodies right down. Imagine they couldn't, and follow what might change for finding food, saving energy and the famous winter sleepers.

Hibernation is a state of greatly reduced metabolism and lowered body temperature that some mammals use to survive winter. A hibernating bat cuts its energy costs by about 98 percent (NPS). This thought experiment removes that option and asks what each animal's winter energy budget might look like, and why scientists still argue about whether bears count.

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The big question

In winter, some animals curl up and sleep for a long, long time.

What if they could not do that? What if they had to stay awake all winter? Let's find out.

When winter comes, a groundhog disappears into its burrow, a hazel dormouse seals itself into a tiny nest and many bats hang quietly in caves. They are hibernating: their hearts slow, their breathing slows and their bodies use much less energy.

What if these animals could not hibernate and had to stay awake all winter? In this thought experiment you can follow three paths: finding winter food, saving energy, and two famous sleepers, bears and dormice.

The National Park Service describes a true hibernator by three changes: a reduced metabolism, a slower heart rate and a lowered body temperature. Britannica defines hibernation as a state of greatly reduced metabolic activity and lowered body temperature that certain mammals adopt as an adaptation to harsh winter conditions.

This thought experiment takes that adaptation away. If bats, groundhogs, dormice and bears had to stay fully active through winter, what would their energy budgets look like? The outcomes are ideas built on measured facts about how much energy hibernation saves.

Explore the idea

Make a prediction

Why do some animals sleep all winter?

What is the main reason some animals hibernate in winter?

According to the National Park Service, which problem does winter dormancy help animals survive?

Food is hard to find, so they save energy
They need time to grow new teeth
Summer is too noisy for sleeping

Established fact
Hibernating animals slow down their hearts and cool their bodies to save energy.

Hibernation slows an animal's heart, breathing and body chemistry so it can last through winter on stored fat.

True hibernators reduce metabolism, heart rate and body temperature; a hibernating bat cuts its energy costs by about 98 percent (NPS).
An idea scientists discuss
A world where animals cannot hibernate is pretend. We imagine what might happen.

No real animal has lost the ability to hibernate. What might happen to them is an idea built from measured facts about how much energy hibernation saves.

The no-hibernation world is hypothetical; its outcomes might differ widely between species and places, depending on food, climate and each animal's other options.
Established fact
In winter there is less food, and it is very cold.

The National Park Service says winter has two big problems for animals: there is less food, and it is intensely cold.

NPS identifies winter's two challenges as reduced food and intense cold; hibernation, migration, food storage and thermoregulation are all responses to them.
Established fact
Animals have different ways to get through winter.

Some animals hibernate, some migrate, like sandhill cranes, and some store food, like honeybees making honey for winter.

NPS lists hibernation, migration (for example sandhill cranes), food storage (for example honeybees) and thermoregulation as winter strategies.
Established fact
When it gets cold, the bugs that bats eat go away.

Bats use a lot of energy flying and must eat a lot of insects. When cold weather drives insects away, bats hibernate or move to warmer places.

Bats have high energy demands from flight; when cold drives insects away they either hibernate or migrate. Hoary bats and Mexican free-tailed bats from Carlsbad Caverns are among those that migrate (NPS).
Established fact
Even in warm places, some bears sleep in winter because food is hard to find.

Black bears as far south as Texas and Florida still go into a winter rest, even without snow or much cold, because food is scarce.

NPS notes that black bears as far south as Texas and Florida go dormant in winter despite little snow or cold, because food is scarce: food, not only cold, drives winter dormancy.
An idea scientists discuss
If bats could not sleep, they might need to fly far away to find bugs.

If bats could not hibernate, more of them might need to migrate to warmer places where insects are still flying.

Without hibernation, species that now hibernate might have to migrate or find a different winter food; whether a given species could do either is an open idea.
Established fact
A sleeping bat's heart beats very slowly, and it uses very little energy.

A hibernating bat's heart rate drops from 200 to 300 beats a minute to 10 beats a minute, which cuts its energy costs by about 98 percent.

During hibernation a bat's heart rate drops from 200–300 to 10 beats per minute, its body temperature can fall to near freezing, and its energy costs fall by about 98 percent (NPS).
Established fact
A groundhog can sleep for months without eating.

A hibernating groundhog's heart slows from about 80 beats a minute to 5. It can go about 150 days without eating and lose no more than a quarter of its weight.

NWF: a hibernating woodchuck's heart slows from about 80 to 5 beats per minute, its breathing from about 16 to as few as 2 breaths per minute, and its body temperature from about 99°F to as low as 37°F; over about 150 days without food it loses no more than a fourth of its body weight.
Established fact
Waking up in winter uses up a bat's saved energy.

Hibernating bats build up a store of fat for winter. If they are disturbed and wake up, they can use up that store before spring.

USGS notes that hibernating bats build a fat reserve to last the winter, and that if they are disturbed the reserve could be exhausted before spring, with fatal results.
An idea scientists discuss
If animals stayed awake all winter, they might need much more food.

Without hibernation, a bat or groundhog might burn through its stored fat long before spring, so it might need to find far more food in the coldest months.

An animal forced to stay active might spend its fat reserve at many times the hibernating rate; unless it could find winter food or move, it might not reach spring. The exact shortfall would differ by species.
Established fact
A sleeping bear does not eat or drink all winter.

At Katmai National Park, a denning bear's heart beats 8 to 10 times a minute, it breathes about once a minute, and it doesn't eat or drink in the den. By spring it may have lost up to a third of its weight.

Katmai reports 8–10 heartbeats and about one breath per minute in hibernating bears, a body temperature that stays above 88°F (31°C), no eating, drinking, urinating or defecating in the den, and weight losses of up to 33 percent by spring.
Established fact
Scientists do not all agree about bears' winter sleep.

Britannica says bears are not true hibernators because their body temperature drops only a little. Yellowstone National Park calls black bears true hibernators.

Britannica excludes bears from true hibernation (little drop in body temperature; easily awakened), while Yellowstone calls black bears true hibernators and grizzlies 'super hibernators'. A 2011 UAF study found black bears cut metabolism to about 25 percent of normal with only a 5 to 6°C drop in body temperature.
Established fact
Hazel dormice sleep in a little round nest on the ground.

Hazel dormice build a small round nest on the ground and seal themselves in. By April about half of them are awake, and by May they usually all are.

PTES: hazel dormice hibernate in a small, sealed, round nest on the ground, where temperatures are steadier; about half are awake by April and nearly all by May. Variable winter temperatures wake them more often, so they can run out of energy.
An idea scientists discuss
If dormice could not sleep, they might find it hard to get enough food in winter.

A dormouse that had to stay awake all winter might not find enough of the hazelnuts, berries and insects it usually eats.

Without hibernation, a dormouse might need a winter food supply it does not have; bears might face the same gap at a much larger scale. These are ideas, not measurements.

In our world

Winter is cold, and there is less food to find.

Some animals fly away to warm places. Some store food. Some go to sleep until spring.

The National Park Service puts it simply: winter brings less food and intense cold. Animals have different ways to get through it. Some migrate, like sandhill cranes that travel through Moab every fall and spring. Some store food, like honeybees that make all their winter honey in spring, summer and fall.

Others hibernate. Before winter, bears eat a lot and turn the food into fat. Then, in their dens, they live on that stored fat for months.

The National Park Service lists several winter strategies: hibernation, migration, storing food and regulating body temperature. Because less food is available in winter, every strategy is a way to obtain or conserve energy.

Hibernators prepare in advance. Larger animals such as bears feed heavily in summer and early fall and convert the food into fat; Yellowstone's bears may gain more than three pounds a day in autumn. Many smaller rodents store food instead. Britannica notes that a hibernator typically relies on reserve body fat, stored food (in rodents only) and a protected den, and that at intervals of several weeks it warms up, wakes, moves about and feeds before returning to torpor.

Imagine a winter with no sleepers

Now imagine the sleepy animals have to stay awake all winter.

They would need to find lots of food in the cold. That might be very hard. It is only pretend!

Imagine that bats, groundhogs and dormice could not slow their bodies down. They would keep using energy at their normal, busy rate all winter long, just when food is hardest to find.

Bats eat insects, and cold weather drives insects away, so a bat that couldn't hibernate might have to migrate to warmer places instead, as some bats already do. A groundhog or dormouse might need to find food all winter, or might run out of its stored fat before spring. These are ideas, not results, but each is based on what scientists have measured.

Remove hibernation and each animal must meet winter's energy cost at its active rate. For a bat, that means paying for the roughly 98 percent of energy costs that hibernation normally saves (NPS), while its insect food is scarce.

Some animals might switch strategy: some bat species already migrate rather than hibernate, and bears in places as warm as Texas and Florida still go dormant because food is scarce. Others might not have a strategy to switch to. USGS warns that a disturbed hibernating bat can exhaust its fat reserve before spring, which hints at how narrow the margin is. How each species might really cope is an idea; it would depend on food, climate and the animal's body.

Why slowing down saves so much

A sleeping groundhog's heart beats very, very slowly.

Its body gets cool. That way it uses only a tiny bit of energy, and its stored fat lasts all winter.

Hibernation saves energy by slowing almost everything down. The National Wildlife Federation says a hibernating groundhog's heart slows from about 80 beats a minute to 5, its breathing from about 16 breaths a minute to as few as 2, and its body temperature can drop from about 99°F to as low as 37°F.

A bat's heart slows from 200 to 300 beats a minute to just 10, which cuts its energy costs by about 98 percent, according to the National Park Service. The National Wildlife Federation adds that a hibernating groundhog can go about 150 days without eating and lose no more than a quarter of its weight.

Metabolic rate, heart rate, breathing and body temperature fall together. NPS reports a hibernating bat's heart dropping from 200–300 to 10 beats per minute, reducing energy costs by about 98 percent; Glacier National Park says marmots shut down to 10 percent of their normal energy use. The National Wildlife Federation gives a groundhog's heart rate falling from about 80 to 5 beats per minute and its body temperature from about 99°F to as low as 37°F, over some 150 days without food.

The extreme case is the Arctic ground squirrel. Researchers at the University of Alaska Fairbanks found its body temperature drops below freezing, the lowest ever measured in a mammal (NPS). Hibernation is not one long sleep: roughly every two to three weeks the squirrel shivers for 12 to 15 hours to rewarm to about 98°F, then cools again. Rewarming takes energy, one reason disturbance matters (USGS).

Bears, dormice and an open question

Bears sleep in dens all winter. They do not eat or drink until spring!

Tiny hazel dormice sleep in a round nest on the ground. Scientists still study how these sleepers do it.

Bears are famous winter sleepers, but scientists disagree about whether they count as true hibernators. Britannica says they are not, because a bear's body temperature drops only a little and bears wake up rather easily. Yellowstone National Park calls black bears true hibernators. At Katmai National Park, a denning bear's body stays above 88°F, it breathes about once a minute, and it doesn't eat or drink until spring.

Hazel dormice build a small round nest on the ground and seal themselves in. The People's Trust for Endangered Species says changeable winter weather can wake them more often, which uses up their energy.

The bear question is a real debate. Britannica says bears are not true hibernators: their body temperature does not fall much and they are easily awakened. One National Park Service article agrees, yet Yellowstone's park pages call black bears true hibernators and grizzlies 'super hibernators'. A 2011 study in Science by Øivind Tøien, Brian Barnes and colleagues at the University of Alaska Fairbanks found black bears slowed their metabolism to about 25 percent of normal while their body temperature fell only 5 to 6°C, cycling between about 30 and 36°C. Their heart rates fell from about 55 to about 14 beats per minute. UAF said the results have implications for treating stroke, heart attack or trauma patients.

For hazel dormice, PTES reports that more die during hibernation than at any other time of year, and that Britain's variable winter temperatures wake them more often, so they can run out of energy.

Amazing facts

The Arctic ground squirrel's body gets colder than icy water while it hibernates.

Scientists at the University of Alaska Fairbanks found that hibernating Arctic ground squirrels cool to below freezing, the lowest body temperature ever measured in a mammal.

Arctic ground squirrels supercool below freezing during 7 to 8 months of hibernation, rewarming every two to three weeks by shivering for 12 to 15 hours (NPS).
A groundhog can sleep for months without eating.

A hibernating groundhog can go about 150 days without eating and lose no more than a quarter of its weight.

Over about 150 days without food, a hibernating woodchuck loses no more than a fourth of its body weight (National Wildlife Federation).
Hazel dormice spend most of the year asleep.

Hazel dormice can spend nearly three-quarters of the year 'asleep' in some form, according to The Wildlife Trusts.

The Wildlife Trusts say hazel dormice can spend nearly three-quarters of the year in some form of sleep; they are usually seen only from April to October.
A sleeping bear's heart beats very slowly.

In one study, hibernating black bears' hearts slowed from about 55 beats a minute to about 14, sometimes with 20 seconds between beats.

In the 2011 UAF study, black bears' heart rates fell from about 55 to about 14 beats per minute; Øivind Tøien reported gaps of up to 20 seconds between beats.

Words to know

hibernation: a long winter rest in which an animal's heart, breathing and body chemistry slow right down and its body cools
metabolism: all the chemical work a body does to turn food into energy and keep itself alive
torpor: a sleep-like state in which an animal's body slows down and cools to save energy
fat reserve: energy an animal stores in its body as fat to use when food is scarce
migrate: to travel from one place to another with the seasons, often to find food or warmth
den: a sheltered home, such as a cave or a hole, where an animal rests or raises young

Discovery challenge

What happens to a bat's heart when it hibernates?

What happens to a bat's heart rate when it hibernates?

According to the National Park Service, how does a bat's heart rate change in hibernation?
- It beats much more slowly
- It beats much faster
- It stays exactly the same
Show the answer

It beats much more slowly

It beats very slowly, so the bat uses very little energy.

It slows from 200 to 300 beats a minute to just 10, saving lots of energy.

It drops from 200–300 to 10 beats per minute, part of a slowdown that cuts energy costs by about 98 percent.
Why do bears in warm Florida still sleep in winter?

Why do black bears in Texas and Florida go into a winter rest even without snow?

Black bears in Texas and Florida go dormant in winter despite mild weather. What does NPS give as the reason?
- Winter food is scarce
- The summer is too hot
- Their fur falls out
Show the answer

Winter food is scarce

There is not much food in winter, so they rest.

Food is scarce in winter, so resting saves energy even where it isn't very cold.

Food scarcity: dormancy is driven by the winter food shortage, not by cold alone.
Where does a hazel dormouse sleep in winter?

Where does a hazel dormouse usually hibernate?

According to PTES, where do hazel dormice usually build their hibernation nests?
- In a small nest on the ground
- High at the top of a tree
- Underwater in a pond
Show the answer

In a small nest on the ground

In a little round nest on the ground.

It builds a small round nest on the ground and seals itself in.

On the ground, sealed in a small round nest, where temperatures are steadier and conditions damp.
What is special about a bear's long winter sleep?

What makes a bear's winter sleep different from a groundhog's?

Britannica does not count bears as true hibernators. Which feature of their winter sleep is the reason?
- Its body stays fairly warm
- It lasts only one day
- It eats all winter long
Show the answer

Its body stays fairly warm

A sleeping bear stays quite warm.

A bear's body stays fairly warm, above 88°F at Katmai, while a groundhog's can cool to as low as 37°F.

Bears show little drop in body temperature and wake easily, though a 2011 UAF study found their metabolism falls to about 25 percent of normal.

Experiments to try

Explore next

For parents & teachers

What children will learn

Winter brings less food and more cold, and animals survive it by migrating, storing food or hibernating.
Hibernation saves energy by slowing heart rate, breathing and metabolism and lowering body temperature.
Hibernators live mostly on fat stored before winter, so waking up too often can use up their reserve.
Scientists still debate how to define hibernation, as the case of bears shows.

Recommended ages: 5 to 17 · About 15 minutes

Talk about it

Why might an animal that can't find food in winter be better off sleeping than searching?
Why do you think people are asked not to disturb hibernating bats in caves?
Should bears count as hibernators? What would you need to know to decide?

Try at home or in class

Count your own heartbeats for 15 seconds while sitting still and multiply by four. Compare your number with a hibernating groundhog's 5 beats a minute (National Wildlife Federation).
Fill two jars with warm tap water. Wrap one in a wool sock or scarf and leave the other bare. After 20 minutes, feel which is warmer and talk about how fur and fat help animals keep in heat.
On a winter walk, look and listen for animals that stay active, such as birds at a feeder, and talk about where the sleeping animals might be.

Key vocabulary: hibernation, metabolism, torpor, fat reserve, migrate, den

Where these facts come from

Hibernation: Nature's Nap Time , National Park Service
Preparing for Winter , National Park Service, Teachers
Bats: Hibernate or Migrate , National Park Service
Bear Hibernation , Katmai National Park & Preserve, National Park Service
Bear Ecology , Yellowstone National Park, National Park Service
Arctic Ground Squirrel , National Park Service
Hibernation-Migration-Fascination Narrative , Glacier National Park, National Park Service
Where do bats live? , U.S. Geological Survey
10 Things You May Not Know About Groundhogs , National Wildlife Federation Blog (Roger Di Silvestro)
How and why hazel dormice hibernate , People's Trust for Endangered Species
Hazel dormouse , The Wildlife Trusts
Hibernation , Encyclopaedia Britannica
During Hibernation, Bear Metabolism Hits New Low (AAAS release on Tøien et al., Science, 2011) , ScienceDaily
Scientists uncover surprising facts about hibernating bears , University of Alaska Fairbanks

Checked, not yet human-reviewed. Last updated October 26, 2026. Spotted a mistake? Tell us.

Picture credits

Photo: U.S. Fish and Wildlife Service Headquarters, public domain, via Wikimedia Commons · source
Photo: I, ElC, CC BY-SA 3.0, via Wikimedia Commons · source · license
Photo: Frank Vassen from Brussels, Belgium, CC BY 2.0, via Wikimedia Commons · source · license
Photo: Cephas, CC BY-SA 3.0, via Wikimedia Commons · source · license
Photo: Oregon State University, CC BY-SA 2.0, via Wikimedia Commons · source · license